In stead of analog broadcasting that broadcasts contents with wireless signals in which analog modulation is applied to the contents, digital broadcasting that broadcasts contents with wireless signals in which digital modulation is applied to the contents starts to prevail worldwide. In digital broadcasting, various characteristic services are provided, such as up-down asymmetrical communications, digital ghost correction, and effective utilization of frequency, which cannot be realized in analog broadcasting. In order to continue contents services provided worldwide in analog broadcasting, most of those contents services are taken over in digital broadcasting. In other words, once digital broadcasting starts, the services in analog broadcasting are almost entirely substituted in digital broadcasting and therefore analog broadcasting service terminates and the frequencies used so far in analog broadcasting can be reused for other wireless communication systems. By using the frequencies used so far in analog broadcasting as well as excess frequency band obtained by improvement in frequency utilization efficiency achieved by digital broadcasting, new broadcasting services not provided in analog broadcasting or fusion of broadcast and communication services are being proposed and put to practice one after another.
Most of these new services were not available in the age of analog broadcasting and are roused by people's new behavior patterns, and many of them are broadcast services in mobile environment or semi-fixed environment using mobile wireless information terminals. Conventionally, broadcasting services for terminals assumed to be fixed (mostly television receivers) are provided by locating a small number of large-scale broadcast stations represented by antenna towers, and those for areas where wireless services are not available from the large-scale broadcast base-station supplemental wired services are provided using RF cables.
Meanwhile, improvement in the operation speed of modern digital signal processing devices is significant and processing speeds equivalent to wireless frequency have already been realized and several times higher processing speeds will soon be realized. In such a high-speed digital signal processing, a technique, called oversampling, of interpolating a digital signal to be processed with a signal of high frequency component and thereby improving the accuracy of the signal processing is disclosed in Japanese Patent Application Laid-open Publication No. 2007-509582, Japanese Patent Application Laid-Open Publication No. 2010-514279, and others.
The oversampling is a process of inserting, between original signals lying on time axis at a first time interval defined by the reciprocal number of a first sampling frequency, a signal at an interval of the reciprocal number of a second sampling frequency that is an integral multiple of the first sampling frequency to interpolate the original digital signals and thereby improve the digital signal processing accuracy. In this interpolation, if a zero signal is inserted at an interval of the reciprocal number of the second sampling frequency, images of the original digital signals are generated as an integral multiple of the original digital signal and lie on the frequency axis. The fact that each frequency spectrum of the each image is the same as the frequency spectrum of the original digital signal is long known in association with the sampling theorem proposed by Shannon in 1948.